feat: add ReAct agent loop and build-time config (mimi_secrets.h)

Rewrite agent_loop.c with ReAct tool use loop: LLM call → tool
execution → repeat until end_turn (max 10 iterations). Add tool
guidance to system prompt. Add set_search_key CLI command.

Add mimi_secrets.h for build-time credentials with highest priority
over NVS/CLI values. All modules (wifi, telegram, llm, proxy,
web_search) check build-time secrets first, fall back to NVS.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main/mimi.c

@@ -20,6 +20,7 @@
 #include "gateway/ws_server.h"
 #include "cli/serial_cli.h"
 #include "proxy/http_proxy.h"
+#include "tools/tool_registry.h"
 
 static const char *TAG = "mimi";
 
@@ -104,6 +105,7 @@ void app_main(void)
     ESP_ERROR_CHECK(http_proxy_init());
     ESP_ERROR_CHECK(telegram_bot_init());
     ESP_ERROR_CHECK(llm_proxy_init());
+    ESP_ERROR_CHECK(tool_registry_init());
     ESP_ERROR_CHECK(agent_loop_init());
 
     /* Start Serial CLI first (works without WiFi) */